The greatest source of ionizing radiation exposure to the general public is from naturally occurring airborne radionuclides inside residences. All substances of natural origin, such as water, rock, soil and construction materials which incorporate crustal materials as aggregate, contain some amount of radium-226 which is a source of radon 222. Radon, being a non-reactive gas, is free to flow through porous soils and, eventually, may enter the environment within a building. Once inside a dwelling, radon will eventually decay into various radon decay products. The initial products in this decay chain series all have very short half lives (less than thirty minutes).
About ninety-five percent of the radon decay products, upon being formed, are positively charged. As a result, they tend to attract other polar molecules in the air, such as water and trace gases, and exist, at least temporarily, as very small and highly diffusive molecular clusters. Such clusters are generally referred to as "unattached" radon decay products. The airborne radioactive decay products also frequently collide with and attach themselves to particles within the air inside a dwelling. In this state, they are referred to as "attached" radon decay products. Such attachment is enhanced by the electrical charge of the radon decay products. Nonetheless, up to about twenty percent of the decay products will generally not attach to airborne particles and will remain in the unattached state, the degree of attachment being heavily dependent on the concentration of particles in the room air.
The health hazard associated with the radon decay series stems from the inhalation and deposition in the lungs of humans of both the attached and the unattached decay products. This is followed by their eventual decay and irradiation of the susceptible lung cell populations. The unattached decay products preferentially deposit in the upper segments of the lungs. This is the site within the lungs where most cancers have been observed among uranium miners who, in the course of their work, were exposed to relatively high concentrations of radon and airborne radon decay products. As a result, the unattached decay products are believed to have the potential for causing a higher localized dose to the lungs (and therefore have a higher associated risk) per unit amount of radioactive material inhaled. In contrast, the larger attached radon decay products are deposited rather uniformly throughout the respiratory system. For this reason, they are considered by radiobiologists to impose a lesser risk (by a factor of as much as 40) of health damage (cancer) per unit amount of radioactive material inhaled by the exposed individuals.